This invention relates to cathode-ray tubes, and particularly to color cathode-ray tubes of the type useful in home television receivers and color display tubes, and to electron guns therefor.
The invention is especially applicable to self-converging tube-yoke combinations with shadow mask tubes of the type having plural-beam inline guns disposed in a horizontal plane, an apertured mask with vertically oriented slit-shaped apertures, and a screen with vertically oriented phosphor stripes. The invention is not, however, limited to use in such tubes and may, in fact, be used, e.g., in dot-type shadow mask tubes and index-type tubes.
An inline electron gun is one designed to generate at least two, and preferably three, electron beams in a common plane and to direct the beams along convergent paths to a small area spot on the screen. A self-converging yoke is one designed with specific field nonuniformities which automatically maintain the beams converged throughout the raster scan.
In one type of inline electron gun, such as that shown in U.S. Pat. No. 3,772,554, issued to R. H. Hughes on Nov. 13, 1973, the main electrostatic focusing lenses for focusing the electron beams are formed between two electrodes referred to as the first and second accelerating and focusing electrodes. These electrodes include two cup-shaped members having the bottoms of the members facing each other. Three apertures are included in each cup bottom to permit passage of three electron beams and to form three separate main focus lenses, one for each electron beam. In such electron guns, static convergence of the outer beams with respect to the center beam is usually attained by offsetting the outer apertures in the second focusing electrode with respect to the outer apertures in the first focusing electrode.
It has been noted that the horizontal beam landing locations of the outer electron beams, in color picture tubes having the above-described electron gun, change with changes in the focus voltage applied to the electron gun. It therefore is desirable to improve such inline electron gun to eliminate, or at least reduce, this horizontal convergence sensitivity to focus voltage changes.
Additionally, there has been a general trend toward inline color picture tubes with greater deflection angles (angles in excess of 90.degree.) in order to provide shorter tubes. In such tubes, it has been found that the electron beams become excessively distorted as they are scanned toward the outer portions of the screen. Such distortions are commonly referred to as flare and appear on the screen of the tube as an undesirable low intensity tail or smear extending from a desirable intense core or spot. Such flare distortions are due, at least in part, to the effects of the fringe portions of the deflection field of the yoke on the beam as it passes through the electron gun, and to the nonuniformities in the yoke deflection field itself.
When the yoke's fringe field extends into the region of the electron gun, as is usually the case, the beams may be deflected slightly off axis and into a more aberrated portion of an electron lens of the gun. The result is frequently a flare distortion of the electron beam spot which extends from the spot toward the center of the screen. This condition is particularly troublesome in self-converging yokes having a toroidal vertical deflection coil, because of the relatively strong fringing of toroidal type coils.
Self-converging yokes are designed to have a nonuniform field in order to increasingly diverge the beams as the horizontal deflection angle increases. This nonuniformity also causes vertical convergence of the electrons within each individual beam. Thus, the beam spots are overconverged at points horizontally displaced from the center of the screen, causing a vertically extending flare both above and below the core of the beam spot.
The vertical flare due to both the effects of the yoke's fringe field in the region of the gun and to the nonuniform character of the yoke field itself is an undesirable condition which contributes to poor resolution of a displayed image on the edge and corners of the screen.
Copending U.S. patent application Ser. No. 461,584, filed on Jan. 27, 1983 to H-Y. Chen and assigned to the assignee of the present invention, discloses a screen grid structure shown in FIG. 8 of the copending application for simultaneously reducing both the sensitivity of the inline electron gun to focus voltage changes and the vertical flare distortion of the electron beam spot. The disclosed screen grid structure utilizes rectangular slots formed in the surface of the screen grid electrode facing the control grid electrode. The slots are aligned with the apertures in the screen grid to create an astigmatic field that produces underconvergence of the electron beam in the vertical plane only, to compensate for the vertical flare distortion. Such a slot structure is described in U.S. Pat. No. 4,234,814, issued to H-Y. Chen et al. on Nov. 18, 1980. The screen grid structure disclosed in the copending Chen patent application also utilizes a pair of reconvergence slots formed on the first accelerating and focusing electrode side of the screen grid electrode. The reconvergence slots are formed closely to and inwardly from the outer apertures in the screen grid electrode and cause a refraction of the electrostatic beam path between the screen grid electrode and the first accelerating and focusing electrode to compensate for the offset refraction within the main lens of the electrode gun. The copending Chen application disclosed a screen grid structure which requires forming two sets of slots on opposite sides of the screen grid electrode. Such a structure is expensive and difficult to manufacture. Thus, a structure which corrects both vertical flare and sensitivity to focus voltage change and is easily and inexpensively produced is desired.